1. Field of the Invention
The present invention generally relates to a laser diode drive circuit and an amplifying circuit for use in an optical disc recording and/or reproducing apparatus. More particularly, this invention relates to a current-to-voltage (I/V) converting circuit for use with a front monitor photodetector (FPD) and to a laser diode drive circuit using the above-mentioned current-to-voltage converting circuit.
2. Description of the Related Art
In recent years, a demand for an optical disc drive apparatus called “super disc drive” capable of reading and writing both a compact disc-recordable/rewritable (CD-R/RW) and a digital versatile disc (or digital video disc)-recordable/rewritable (DVD-R/RW) has been increasing on the market.
The optical disc drive apparatus of this kind generally uses a power monitor circuit to detect light intensity of laser beams. This power monitor circuit includes a monitor diode to receive several percents (this numerical value changes depending upon design of optical system) of laser beams emitted from a laser diode (LD). In that case, when the power monitor circuit includes only one laser diode LD, a change in quantity of incident light due to a fluctuation of a divergent angle of the laser diode LD or due to a fluctuation of a beam splitter is within a range of approximately ±6 dB.
When the change in the quantity of incident light falls within the above-mentioned range, it is possible to adjust the conversion resistance value of the current-to-voltage converting circuit of the front monitor photodetector circuit in response to dispersion of individual operational amplifiers without sacrificing the performance such as an operation band, offset, or the like.
However, let us consider now the electric power necessary for the two standards of the recordable/rewritable compact disc (CD-R/RW) and the recordable/rewritable digital versatile disc (DVD-R/RW). When speed at which data is to be recorded in the CD-R/RW is assumed to be 16 times as high as the normal speed, power (electric power) required by the laser diode LD becomes 160 mW expressed in the form of laser diode radiation electric power, and electric power required by the laser diode LD to record data in the DVD-R/RW becomes 70 mW expressed in the form of laser diode radiation electric power. Having compared two center values of the above-mentioned laser diode radiation electric power, it is to be understood that they have already differed from each other more than twice.
Further, having considered dispersion of individual laser diodes LD in addition to the difference of the laser diode radiation electric power, a range greater than ±12 dB is required as the range in which a parameter of a circuit is to be adjusted. Therefore, when it is intended to secure such a wide range for adjusting a parameter of a circuit from a circuit design standpoint, it becomes impossible to maintain the performance of the laser diode LD, such as the offset.
In order to realize the laser diode drive apparatus capable of reading and writing both the above-mentioned CD-R/RW and the above-mentioned DVD-R/RW, as shown in FIG. 1, there are required two high-output laser diodes LD1 and LD2 generating laser beams having wavelengths of 650 nm and 780 nm respectively. It is possible to realize functions necessary for the laser diode drive apparatus, even when an operational amplifier is configured in such a manner that the LD1 and LD2 are in separate optical systems. However, in this case it becomes unavoidable that the laser diode drive apparatus becomes large in size and becomes very expensive.
Accordingly, a demand for an operational amplifier having as many components other than the laser diode LD in common as possible is increasing.
FIG. 1 of the accompanying drawings is a schematic circuit diagram in block form, which shows a laser diode drive circuit for use in an optical disc recording and/or reproducing apparatus according to the related art. As shown in FIG. 1, this related-art laser diode drive circuit is comprised of a laser diode drive circuit, a power monitor circuit and an automatic power control (APC) circuit, whereby the two laser diodes LD1 and LD2 generating the laser beams having the wavelengths of 650 nm and 780 nm can be controlled respectively.
As shown in FIG. 1, this APC circuit has a configuration of using elements and parts in common in the circuit necessary for the two laser diodes LD1 and LD2, and only electric power is switched over to be set. Similarly, a laser diode drive power supply circuit LDD can be provided with two output terminals used to switch-over an output and other portion (power monitor circuit) of the laser diode drive circuit can be used in common.
The power monitor circuit for the disc writing laser diodes LD1 and LD2 is adapted to receive and monitor several percents (this numerical value changes depending upon design of the optical system) of emitted power of laser beams from the laser diode LD1 or LD2 by a monitor diode PD. In that case, when the power monitor circuit includes only one laser diode LD, a change in quantity of incident light due to a fluctuation of a divergent angle of the laser diode LD or a fluctuation of a beam splitter is within a range of approximately ±6 dB. When the change in the quantity of incident light falls within the above-mentioned range, it is possible to adjust the conversion resistance value of the current-to-voltage converting circuit of the front monitor photodetector (FDP) circuit in response to dispersion of individual operational amplifiers without sacrificing the performance such as an operation band, offset, or the like.
However, when electric power necessary for writing data in two standards of the CD-R/RW and the DVD-R/RW is considered, power (electric power) required by the laser diode LD necessary for the CD-R/RW becomes 160 mW and power required by the laser diode LD in the DVD-R/RW becomes 70 mW expressed in the form of laser diode radiation electric power at drive speed of the CD-R/RW being 16 times as high as the normal speed. Accordingly, two center values of the above-mentioned laser diode radiation electric power have already differed from each other more than twice. Further, having considered dispersion of individual laser diodes LD in addition to the fluctuations of the laser diode radiation electric power, a range greater than ±12 dB is required as the range in which a parameter of a circuit is to be adjusted. Thus, it becomes difficult to maintain the performance of the laser diode LD, such as the offset satisfactorily.
Furthermore, if the dynamic range of the optical disc recording and/or reproducing apparatus is doubly changed with respect to the CD-R/RW or the DVD-R/RW, then the available power margin is narrowed unavoidably. As a result, setting accuracy on the DVD-R/RW side where power should be set with high accuracy is doubly degraded, which is not preferable.